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I’d like to be one up on the mechanics at the dealer when they try to smooth things over and tell me this was no big deal.
It would be nice to know what year Outback you have.
The sound seems to be the exhaust system cooling down. It may be fixable in a few seconds or it may mean replacing a part or two.
Hint. You can find exactly where it is coming from by using an length of garden hose as a stethoscope.
It really is not likely to be a big deal. Likely it is only the noise and may be fixable fast and easy. I doubt if I would worry about it since I generally don’t listen to my car cool down. 
However if it bother’s you and IF your car is still under warranty, then you are entitled to have it corrected or reduced. You may want to research your state’s lemon laws to know where you stand legally and what you need to do to document the problem.
I agree with Mr. Meehan that this is likely just the uneven contraction of metal parts in the exhaust system as they cool down. While it can certainly be annoying, luckily it is not something that affects the safety or the durability of the vehicle. I have had cars that exhibited this syndrome, although I have to say that the noise did not persist for 5 minutes, as you say yours does.
This is something that the dealer may be able to remedy under the terms of the various warranties–if this is a new vehicle. You have not given us any details regarding the age of the vehicle.
If this is not remedied under warranty, as Mr. Meehan suggests, you need to check the details of the Lemon Law in your state to see if this is something that might be covered. For instance, in my state, the statute covers “a defect or condition which substantially impairs the use, value or safety of a motor vehicle”, and if your state’s statute is similar, then the question arises of whether a non-destructive noise from a parked vehicle actually impairs the use, value, or safety of the vehicle.
Playing Devil’s Advocate for a moment, I think it could be argued that a non-destructive noise from a parked vehicle is not something that impairs your use of the vehicle or its safety. However, it could slightly–but not substantially–impair its value if you were to try to sell it. I just want you to be prepared for the possibility that this problem may not be covered by the Lemon Law in your state.
If this is not a new vehicle, then you are likely not protected under a Lemon Law.
If this is a brand-new vehicle, it is possible that the problem will resolve itself to a certain extent after a few weeks. Not that it is likely to disappear, but it may be less noticeable after a few weeks.
Thanks for the input thus far – yes, this vehicle has 250 miles on it! And the problem didn’t start until about mile 235.
New car with all the seals and metal parts trying to “get used to each other”. Your call, but you will have the warranty for a while. I say let it go and see if it gets better. I don’t think, them going at your new car with the tools is going to be helpful and might actually cause some damage.
Thanks for all your advice. The final verdict is that this is the catalytic converter breaking in – pretty much what you guyz suggested. Guess I’ll have to get used to the noise. If I hadn’t heard the same from you guyz, though, I’d have been suspicious with the outcome as told to me by the dealer. So thanks for your help.